1. Field of the Invention
The present invention relates to a substance that can emit light by current excitation. In particular, the present invention relates to a substance that can obtain light emission from a triplet excited state. In addition, the present invention relates to a light-emitting element and a light-emitting device using the substance.
2. Description of the Related Art
A light-emitting element using an organic compound has characteristics such as a thin shape and lightweight, and high-speed response. Additionally, the light-emitting element is self-light emitting element. Therefore, a display device using the light-emitting element for a pixel portion has been actively developed in recent years.
A light-emission mechanism of the light-emitting element is said as follows. By applying a voltage by sandwiching a light-emitting layer between a pair of electrodes, electrons injected from a cathode and holes injected from an anode are recombined at a light-emission center of the light-emitting layer to form a molecular exciton, and energy is released from the molecular exciton in returning to a ground state, thereby emitting light. A singlet-excited state and a triplet-excited state are known as an excited state, and it is considered that light emission is possible through either excited state.
In such a light-emitting element, since more of triplet-excited states are generated than a single-excited state, luminous efficiency of the light-emitting element can be increased by using a material that can emit light from a triplet-excited state (a phosphorescent material). Therefore, it has been attempted number of times so far to use a phosphorescent material for a light-emitting element.
There is a metal complex, where iridium (Ir) is the central metal (hereinafter, referred to as an Ir complex), as a typical phosphorescent material which emits green light (for example, see Reference 1: M. A. Baldo and four others, Applied Physics Letters, Vol. 75, No. 1, p. 4). In Reference 1, green light emission is obtained by dispersing the Ir complex, where 2-phenylpyridine is a ligand, into a host material.
However, most of the phosphorescent materials generally emit light having a comparatively long wavelength such as red or orange light, and there are a few reports of a phosphorescent material that emits green or blue light so far. As for an Ir complex where 2-phenylpyridine and a derivative thereof are ligands, it is known that light having a wavelength band of green to blue is emitted. However, there is a property that holes are likely to be injected, whereas electrons are unlikely to be injected; therefore, an element structure thereof is limited in a case of applying the Ir complex to a light-emitting element. Moreover, there is also a problem that the Ir complex is poor in heat resistance, which can be said for the overall organometallic complexes.
Therefore, in the case of applying a phosphorescent material to a light-emitting element, it has been required to develop various phosphorescent materials which emit light having a wavelength band of green to blue so that the phosphorescent material can respond to a combination with various peripheral materials such as a host material, a hole-transporting material, and an electron-transporting material. In addition, it has been required to develop a phosphorescent material of green light or blue light having high heat resistance.